Symfony\Component\EventDispatcher\EventDispatcher::filter PHP Method

    public function filter(Event $event, $value)
    {
        foreach ($this->getListeners($event->getName()) as $listener) {
            $value = call_user_func($listener, $event, $value);
        }

        $event->setReturnValue($value);

        return $event;
    }

 public function testFilterWhenThereIsNoEsiIncludes()
 {
     $dispatcher = new EventDispatcher();
     $listener = new EsiListener(new Esi());
     $dispatcher->connect('core.response', array($listener, 'filter'));
     $event = new Event(null, 'core.response', array('request_type' => HttpKernelInterface::MASTER_REQUEST));
     $dispatcher->filter($event, $response = new Response('foo'));
     $this->assertEquals('', $response->headers->get('Surrogate-Control'));
 }